Unsaturated polyester resins are generally formed by polymerising together an unsaturated dihydric alcohol, such as ethylene or propylene glycol, with an unsaturated dicarboxylic acid, such as maleic acid, fumaric acid or phthalic acid, or mixtures thereof. The polymerisation reaction is generally quenched by pouring the reaction mixture at a temperature of about 200.degree. C. into system maintained at a temperature in the range 30.degree.-80.degree. C. The concentration of the resin in the styrene solution is generally about 70% by weight at this point, but further styrene is added to dilute the solution to about 50-5% by weight of resin for ease of handling in commercial applications. The styrene solution of the resin at a concentration in the range 50-55% by weight has almost Newtonian rheological properties so that when the resin is used to form a fibre-reinforced structure the solution tends to flow under gravity on vertical or inclined surfaces. It is customary, therefore, to incorporate into the styrene solution of the resin a thixotropic gelling agent. One gelling agent which has been successfully used is fumed silica, which may be stirred into the resin solution using moderate shear. However fumed silica has the disadvantages that its bulk density is very low, being of the order of 0.06 g.cm.sup.-3 and that it generates hazardous dust. Its cost is also relatively high, the cost of the fumed silica being about 4% of the selling price of the resin solution.
Organoclays provide a possible alternative to fumed silica and have the advantages of being cheaper and less dusty than fumed silica and of having a higher bulk density in the range of from 0.3 to 0.8 g.cm.sup.-3. However, organoclays generally have the disadvantage that they will not disperse and provide the required gelling properties if stirred directly into the solution of the polymer in styrene. It is necessary first to disperse the organoclay in styrene to form a "pregel" and then to mix the pregel with the polymer solution. An additional mixing stage is therefore required and the additional capital cost is not favoured by manufacturers. A further disadvantage is that, since the concentration of polymer in the styrene should be as high as possible, the quantity of styrene which can be added in the form of a pregel is limited and the highest concentration of organoclay in styrene which can be achieved before the suspension becomes undesirably viscous and difficult to handle is about 5% by weight. In cases therefore where a particularly viscous polyester resin composition is required, it may not be possible to use an organoclay as the thixotropic gelling agent.
The object of the invention is to provide an organoclay composition which can be stirred directly into a styrene solution of a polyester and which will provide thixotropic gelling properties.
British patent application No. 85.31254 corresponding to U.S. Ser. No. 940,988 of which this application is a continuation-in-part, describes a thixotropic gelling composition for unsaturated polyester resins which comprises a mixture of a smectite clay, a quaternary ammonium compound which is capable of rendering the smectite clay organophilic and a particulate second inorganic material, in particular finely divided silica having an average particle diameter in the range from 0.005 .mu.m to 0.5 .mu.m, the second inorganic material constituting from 10% to 50% by weight of the mixture. There is preferably used as the quaternary ammonium compound a mixture comprising from 25 mol % to 100 mol % of dimethyl benzyl hydrogenated tallow ammonium chloride (2MBHT) and from 75 mol % to 0 mol % of dimethyl di (hydrogenated tallow) ammonium chloride (2M2HT). The mixture of smectite clay, quaternary ammonium compound and second inorganic material in suspension in water is preferably subjected to high shear mixing for a time sufficient to dissipate in the mixture at least 100 KJ of energy per kg. of dry solids in the mixture.
U.S. Pat. No. 4,473,675 describes a thixotropic cross-linkable unsaturated polyester composition which comprises an aromatic unsaturated monomer having an amount of an organoclay dispersed therein. The organoclay comprises the reaction product of a smectite clay and a mixture of 2M2HT and 2MBHT, the amount of quaternary ammonium compounds used being from 95 to 140 milliequivalents per 100 g of dry clay.
British patent No. 2,090,584 describes an organoclay composition which can be stirred directly into a solution of a polyester in styrene (page 32, lines 23-27 and 52-55), but the organoclay, in addition to the quaternary ammonium compound, also requires from 5-100 milliequivalents per 100 g of dry smectite clay of an organic anionic compound (page 5, lines 20-22).